The United Nations has launched the largest appeal in its history - seeking $5bn (£3.2bn; 3.7bn euros) for humanitarian aid to Syria while warning of a regional "explosion" if the foreign-backed insurgency does not stop.

The world body said Friday that a total of $3.8 billion is needed to help Syrian refugees who have spilled across the country's borders to escape fighting at home.

The figure for operations inside Syria was put at another $1.4 billion.

"If the fighting doesn't stop, we risk an explosion in the Middle East for which the international community is not prepared," UN refugee agency head Antonio Guterres told reporters.

"It is not only a matter of generosity but also of enlightened self-interest."

Thousands of people have been killed and some 1.6 million Syrians fled the country since the bloody foreign-backed insurgency began in March 2011.

The number of refugees is expected to reach at least 3.45 million by the end of this year, according to the UN appeal.

Within the country, 6.8 million people are forecast to need aid this year, the majority of them having been forced to flee their homes because of the fighting.

The UN was also scrambling to find replacement troops for its peacekeeping mission on the occupied Golan Heights after Austria announced it was pulling out its 377 troops from the 1,000-strong UN Disengagement Observer Force.

UNDOF peacekeepers from the Philippines and India were wounded by mortar shrapnel in fighting for the strategic crossing, according to UN diplomats.

Manila said it too was considering pulling out its 341-strong contingent.

Russia suggested on Friday that it is ready to replace Austria in the crossing.